president obama was notified within minutes. he promised action. >> i'm supremely confident that bostonians will pull together, take care of each other and move forward as one proud city, and as they do, the american people will be with them every single step of the way. >> reporter: officials are tracking multiple persons of interest including a man being treated at an area hospital, as well as another individual who was seen trying to gain access to a restricted area five minutes before the blasts. john and diana. >> all right, tahman bradley, live in boston, thank you. >>> well, the fbi is launching a massive effort to find those responsible for the deadly attack. they're going to be looking at bomb fragments. they'll also look at hundreds of videos from the finish line. >>> with more on the search for clues, we're joined by abc's devin dwyer from washington. good morning, devin. >> reporter: good morning. the boston marathon attack is the first successful bombing in the u.s. since the september 11th, 2001 attacks. now the fu